In a duplicate image forming device for reading an original image from an original sheet, and forming a duplicate image according to the original image, such as an electrographic copying device and a thermal stencil master plate making/printing device, conventionally, a plurality of duplicate images or duplicate stencil master plates are formed or prepared from a single original sheet for two-color or multi-color copying and stencil printing.
For instance, when only a part of the original image on an original sheet is to be stencil printed in a different color, two stencil master plates are prepared, one of them including only a designated region of the original image that is to be printed in one color while the other stencil master plate covers the remaining part of the original image that is to be printed in another color, and the two stencil master plates are used one after the other using inks of two different colors on a same sheet of printing paper.
In the case of preparing a duplicate stencil master plate, a duplicate stencil master plate may be prepared for each of a plurality of original sheets each covering a different region of an original image, but it means a substantial complication in the preparation of the original sheets because the same number of original sheets as the number of necessary colors are required.
It is known to designate a region of an original image that is to be extracted from the original image or an ineffective region of the original image that is to be discarded by using a coordinate input device such as a digitizer, and to carry out the process of preparing a stencil master plate only with respect to the effective region which may be either the designated region or the area other than the designated region. In this case, all that is needed is a single original sheet which includes all of the original image to be printed, and there is no need to prepare a same number of original sheets as the number of colors.
However, even when a digitizer is used, a region of the original image that is to be extracted or a region of the original image that is to be made ineffective must be designated with the coordinate input means each time a stencil master plate is to be prepared, and this requires not only a substantial amount of work but also introduces the possibility of erroneously designating an area.
In particular, when different regions of an original image are designated from an original sheet and are individually read, because the designation of a region with a digitizer and the reading of the original image are carried out in alternating turns, it is necessary to mount the original sheet on the digitizer every time a stencil master plate is to prepared.
This is not only inconvenient for the operator but also may cause some relative errors to be produced for each stencil master plate because the original sheet may not be properly registered every time it is mounted on the digitizer for designating a region. Thus, the process of plate making and test printing must be repeated a number of times before a satisfactory result can be obtained.
This problem arises not only in the process of preparing duplicate stencil master plates but also in the process of forming duplicate images such as image copying.